The government has confirmed that it will cut the number of MPs in the House of Commons from 650 to 600, rejecting a report by a cross-party committee of MPs that described the move as unjustified.

Responding to a report by the Commons political and constitutional committee published last March, the government said it saw “no merit in reopening the issue”, and it would press ahead with plans to reduce the number of MPs in parliament and to ensure that no constituency contained more than 5% above or below the national average number of voters.

The size of Westminster constituencies varies wildly, ranging from 21,769 voters in Na h-Eileanan an Iar (the Western Isles) to 108,804 in the Isle of Wight. The Conservative party manifesto contained a pledge to equalise the size of constituencies in order to “cut the cost of politics and make votes of more equal value”.

Studies have suggested that the changes could result in the Conservatives gaining up to 20 more seats in parliament, and Labour politicians have accused the government of pushing through the changes for party political gain.

The 2010 coalition agreement pledged the “creation of fewer and more equal-sized constituencies”, and the Parliamentary Voting System and Constituencies Act 2011 subsequently made provision to reduce the size of the House of Commons to 600 seats.

A review of constituency boundaries was due to be completed before last May’s general election, but in January 2013 MPs voted to delay the decision. The new boundary review begins this year and will be published in 2018, with the new constituency borders implemented by the 2020 general election.

The political and constitutional committee’s report argued that numbers of MPs should be whittled down gradually, rather than abolishing 50 constituencies in one go, and that ministers should allow a 10% deviation from the mean number of constituents.

The response from the Cabinet Office said a 10% leeway would be “too large and would undermine the basic principle of equally sized constituencies”.

It said: “Within the rules set out by the act, commissions can continue to take into account factors such as physical geographical features including mountains and rivers, local government boundaries and local ties, and the boundaries of existing constituencies.

“However, these factors are subject to the overriding principle of equality in constituency size, because the government remains of the view that equality and fairness must be paramount.”